1. Field of the Invention
This invention relates to heavy duty pneumatic radial tires, and more particularly to a belt structure of a heavy duty pneumatic radial tire such as off-the-road radial tire for construction vehicles which can improve a durability to belt end separation generated in the belt arranged radially outside a carcass ply when the tire is run on rough road at a high speed.
2. Related Art Statement
Heretofore, in order to restrain the belt end separation generated when the tire is run on rough road at a high speed, there have been known a structure wherein plural main belt layers 2 each containing substantially inextensible steel cords are arranged radially outside carcass plies 1 as shown in FIGS. 7 and 8 and an HE (high elongation) steel cord layer 3 having a width wider than those of the main belt layers is arranged on the main belt layers 2 to protect the belt ends 2a of the belt layers 2, a structure wherein a reinforcing layer 4 composed of fiber cords is placed on the belt ends 2a of the belt layers 2 as shown in FIG. 9, and the like.
In the structure of FIG. 8, however, the separation due to cracks generated at the belt end of the main belt layer 2 composed of substantially inextensible steel cords is restrained to some extent by the wide HE cord layer 3 acting as a protect belt layer for the main belt layer, but when a large input is locally applied to the tread through the riding on rocks or stones, since the width of the wide HE cord layer 3 becomes wider, a large strain is generated at the end portion of the wide HE cord layer 3 and hence cracks are produced in rubber in the vicinity of the end of the wide HE cord layer 3, resulting in the formation of nucleus for the separation failure. On the other hand, the structure of FIG. 9 mainly intends to suppress the size expansion through centrifugal force acting to the tread during high speed running as a passenger car tire and control the strain at the belt end and is generally composed of a nylon cord layer extending substantially in parallel to the circumferential direction, but is not so effective for suppressing a large strain locally produced at the belt end during the running. For example, the tire of this type having a tire size of 36.00 R51 is run on scattered rocks under a super-heavy load of about 46 tones per tire.